The present invention relates generally to a Light Emitting Diode (LED) driver and, more specifically, to an LED driver having an extended dimming range.
Recent developments of high-brightness light emitting diodes (LED) have opened new horizons in lighting. Highly efficient and reliable LED lighting continuously wins recognition in various areas of general lighting, especially in areas where cost of maintenance is a concern.
A wide dynamic range of the LED brightness control becomes important in many applications, such as automobiles, avionics and television. In some cases it is needed due to large variation in the ambient light, in others it allows to improve the contrast ratio of a display. Due to the color and chromaticity properties of LED's, it is beneficial to control brightness of an LED through pulse width modulation of the current in it, while maintaining the current magnitude at a fixed level. This LED brightness control method is commonly referred to as the PWM dimming.
Presently, the brightness control range of current circuits is limited to the minimum on time of a switch needed to maintain the current magnitude in the LED string. When the output pulse width of a generator becomes shorter than the on-time of the switch needed for the current sense voltage to reach the error voltage level, the control over the LED string current is lost, and the current drops out of regulation. This limit is more restrictive, when an inductor is operated in continuous conduction mode (CCM), since a longer time is needed for it to develop its steady-state current.
Therefore, it would be desirable to provide a circuit and method that overcomes the above problems.